Polyurethane foam is demanded to have various mechanical characteristics including heat insulating property, vibration absorbing characteristics and the like depending on purposes, and particularly in a vehicle seat pad and the like, comfort, such as comfort to sit and high repulsive elasticity, is demanded. For aiming enhancement of the comfort, the structure of the main component of the polyurethane foaming raw material and the subsidiary raw material therefor, such as a filler, have been reconsidered for enhancing the vibration absorbing characteristics around 6 Hz (ca. from 4 to 8 Hz), which is ordinarily said to be the resonance point with the internal organs of human body, and such attempts have been made for decreasing the resonance frequency and the resonance magnification of the resulting polyurethane foam. For decreasing the resonance frequency and the resonance magnification, it has been effective that the density is increased, and the weight is increased.
A vehicle seat pad may often suffer such a problem that the seat fails to support the human body due to the centrifugal force applied to the body on changing lanes or driving a curve, thus shaking the body. For reducing the shakiness, such measures have been made that (1) the molecular weight of polyether polyol used as a raw material of polyurethane foam is changed from a high molecular weight to a low molecular weight, (2) the proportion of the ethylene oxide unit in the repeating units derived from ethylene oxide and propylene oxide in the polyether polyol is increased, (3) the water content in the foaming liquid for forming polyurethane foam is increased, and (4) the amount of the crosslinking agent is increased.
On taking the measures, however, there may be a problem that the resulting polyurethane foam is lowered in repulsive elasticity and air permeability, and the stress relaxation, which is important in ride comfort, is deteriorated (increased). That is, there is a trade-off between the shakiness and the stress relaxation, i.e., when the shakiness is improved (lowered), the stress relaxation is deteriorated (increased), and when the stress relaxation is improved, the shakiness is deteriorated.
A vehicle is demanded to be enhanced in fuel efficiency due to increase of consciousness on environmental loading reduction in recent years, and as part thereof, the constitutional parts have been reduced in weight. The attempts of weight reduction is extended to all the constitutional parts including the exterior and interior of the vehicle, and also to a seat pad. However, when the density of the foamed material is decreased, there may be such a problem that the vibration absorbing characteristics, which influence the comfort, are deteriorated, and thus it is difficult to achieve both the density reduction and the vibration absorbing characteristics. Accordingly, in view of the needs including the ride comfort and the enhancement of fuel efficiency, such a seat and a cushioning material are demanded that have light weight and suitable repulsive force and vibration absorbing characteristics.
According to the demands, the present inventors propose polyurethane foam containing a polyurethane foaming liquid containing a polyol component and an isocyanate component as main components, having been foam-molded, in which the polyol used is a polyether polyol having a molecular weight of from 3,000 to 12,000, an unsaturation degree of 0.03 mEq/g or less and a ratio, molecular weight/number of functional group, of from 1,000 to 3,000, and an organized inorganic filler is mixed therein (see Patent Document 1). The polyurethane foam proposed therein has lightweight and is excellent in vibration absorbing characteristics.